The present invention relates to an optical signal processing apparatus and, more particularly, to an optical signal processing apparatus including a light-receiving circuit which receives and processes a high-speed digitally modulated optical signal used in an optical communication system or the like, an optical signal processing apparatus including the light-receiving circuit and a band limiting circuit which limits the band of a digital signal processed by the light-receiving circuit, and an optical signal processing apparatus including the light-receiving circuit and an error measuring circuit which measures an error of an original high-speed digitally modulated optical signal on the basis of an output from the light-receiving circuit or the band limiting circuit.
When, e.g., an error of the above-described high-speed digitally modulated optical signal used in the optical communication system or the like is to be measured, the correlation between the intensity of input light and the error rate must be examined.
For this purpose, an optical signal error measuring circuit having an arrangement as shown in FIG. 13 has conventionally been used.
More specifically, an optical fiber 1 for transmitting a target measurement optical signal is connected to a light-receiving circuit 2 including a photodiode 2a.
A light reception signal from the photodiode 2a is output to an error measuring device 3 to measure an error of the optical signal.
Then, the connection of the optical fiber 1 is changed from the light-receiving circuit 2 to an optical power meter 4 including a photodiode 4a.
The intensity of the optical signal is measured by a light reception signal from the photodiode 4a.
In an arrangement shown in FIG. 14, an optical signal output from an optical fiber 1 is branched by a photo coupler 5. Then, one branched signal is input to an error measuring device 3 through a light-receiving circuit 2 including a photodiode 2a, and the other is input to an optical power meter 4 including a photodiode 4a. The error rate and intensity of the optical signal are simultaneously measured on the respective paths.
In, however, the method of performing measurement upon changing the connection of the optical fiber 1, as shown in FIG. 13, the connection state of the optical fiber 1 undesirably varies, resulting in inaccurate measurement.
In the method of branching the optical signal from the optical fiber 1 by the photo coupler 5, as shown in FIG. 14, the intensities of light input to the light-receiving circuit 2 and the optical power meter 4 greatly decrease upon branching, decreasing the S/N. As a result, an optical signal with a low intensity cannot be accurately measured.
Either method described above requires two light-receiving circuits, resulting in a high-cost measuring system as a whole.
To solve these problems, it can be considered to amplify a light reception signal from one photodiode, and branch the amplified output to two systems.
This method however requires a very-high-cost amplifying circuit which has high-speed response characteristics corresponding to a high-speed digitally modulated signal, and a wide dynamic range enough to allow measurement of power in a wide input range.
Jpn. Pat. Appln. KOKAI Publication No. 6-37556 discloses a prior art of detecting two branched signals by using the two terminals, i.e., anode and cathode of one photodiode.
In this prior art, load resistors are respectively connected to the anode and cathode of the photodiode to reverse-bias the photodiode.
In this case, however, a voltage drop caused by the load resistor becomes large for a received optical signal having a large magnitude. As a result, the voltage across the two terminals of the photodiode decreases to degrade the response characteristics.
Further in this prior art, it is basically impossible to couple the photodiode to a DC amplifier in a DC manner to apply a reverse bias. A DC-cut capacitor must be inserted.
This fact is derived from the fact that DC amplification is impossible.
That is, this prior art has a problem that a response from a DC component is impossible because the DC component of an input to the DC amplifier must be cut.
On the other hand, in the optical communication system as described above, high-speed digital signals of 52 Mbps, 156 Mbps, and 622 Mbps are used in a transmission system for transmitting high-speed digitally modulated signals as multiplexed optical signals.
In general, in an optical signal error measuring device which receives and processes such a digital signal from the light-receiving circuit as described above, amplifying devices each having a band limited for a corresponding bit rate are selectively used in accordance with the bit rate of an input digital signal.
The use of the amplifying devices independent for the respective bit rates as described above complicates the arrangement of the whole measuring system, resulting in an increase in mounting area. The whole measuring system cannot be downsized.
To solve this problem, it can be considered that band limiting filters corresponding to respective bit rates are arranged to be selectively connected to a signal path in one amplifying device by using a relay switch or a diode switch, thereby switching the band in accordance with the bit rate of an input digital signal.
This method however requires, on the input and output sides, a plurality of filter circuits independent for the respective bit rates, and relay switches for switching these filter circuits. The apparatus cannot be sufficiently downsized, and the reliability is degraded by the use of the relay switches.
In a method of selectively connecting a signal path and each filter by a diode switch in place of the relay switch, a digital signal is distorted by the nonlineality of the diode.